Body mass index is your weight divided by the square of your height, and it is one of the few longevity-relevant numbers you can obtain entirely at home — a scale, a wall and a tape measure are the whole kit. That accessibility is why it is used everywhere, and why it is over-read: BMI describes how heavy you are for your height, and nothing more. It was devised in the 1830s to describe populations, not individuals, and it still works better on a group than on a person.
What you need
- A bathroom scale. Its absolute calibration matters less than using the same one every time.
- A bare wall, a hard flat floor, a book, a pencil and a tape measure.
- Your weight in kilograms and your height in meters, or pounds and inches — only the conversion changes.
- Your age and sex — not inputs to the arithmetic, but necessary to interpret the answer.
- Your population background, because the action points recommended for many Asian populations sit lower than the standard ones.
How the measurement works
Height. Stand without shoes, heels together against the wall, back and shoulders touching it, looking straight ahead rather than up. Rest the book flat on the crown of your head, mark the wall at its underside, and measure from the floor. Height falls slightly through the day as the spinal discs compress, so repeat measurements at the same hour.
Weight. Weigh in the morning, after using the bathroom and before eating, in minimal clothing, on a hard floor rather than carpet.
The arithmetic. Weight in kilograms is divided by height in meters multiplied by itself: at 1.75 m and 75 kg, that is 75 divided by 3.0625. Squaring the height matters, because an error there counts twice over — a one percent mismeasurement of height moves BMI about twice as far as the same error in weight. To skip the conversion and the squaring, Body Mass Index (BMI) takes height and weight in either unit system and returns the value with its category.
How to read the result
The World Health Organization classification for adults, in kg/m²:
| BMI | WHO category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal range |
| 25.0 to 29.9 | Overweight (pre-obesity) |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
These bands apply to adults. Children and adolescents are assessed against age- and sex-specific BMI-for-age percentile charts instead; the adult labels do not transfer. A WHO expert consultation also recommended lower public-health action points for many Asian populations, commonly cited at 23 and 27.5 kg/m², because cardiometabolic risk appears at a lower body mass.
Treat the boundaries as lines drawn through a continuous variable, not biological switches: a BMI of 24.9 and one of 25.1 are the same body. For one person, the direction of travel over years — measured the same way each time — carries more information than the label.
What it does not tell you
BMI does not measure fat. It cannot tell muscle, bone and water from adipose tissue, so a trained strength athlete is routinely labeled overweight while carrying little fat, and someone sedentary with low muscle mass can sit in the normal range with a high fat percentage — the pattern sometimes called normal-weight obesity. A direct estimate such as Body fat assessment answers a different question.
It also says nothing about where the fat sits. Two people at the same BMI can differ completely in visceral fat, the depot most closely tied to metabolic risk. A waist-based measure covers that gap for the price of one more tape reading; Waist-to-height ratio (WtHR) is the usual companion because it needs no population-specific threshold.
Age distorts it in both directions. Adults lose height from midlife onward, which raises BMI on unchanged weight, and muscle is often lost as fat is gained, so a stable BMI can hide a substantially changed body. It is also unreliable in pregnancy, after limb loss, and with fluid retention or edema, where weight reflects water rather than tissue.
Finally, BMI is not a health assessment. It contains no blood pressure, no glucose, no lipids and no fitness, and clinical guidance increasingly treats it as a first screening step, to be confirmed by a direct measure of adiposity rather than acted on alone.
Related reading
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.